This invention relates to a facsimile transmitter for generating transmission signals representing dark/light variations in a document located at the facsimile transmitter.
Facsimile transmitters typically utilize a communications link in the form of a telephone network to transmit the signals representing dark/light variations in a document to a remotely located receiver. In many if not most instances, the facsimile transmission is uneventful and the receiver receives the transmission signal and converts those signals into dark/light variations on a copy medium without interruption. However, in some instances the receiver may not be able to continue a transmission for a variety of reasons including equipment failure, operator failure or operator choice. If a receiver does discontinue reception of the transmission signals, it is extremely undesirable to continue transmitting signals from the transmitter since such a transmission will consume a substantial length of time which can be rather costly where a communication link such as the telephone network is involved. For example, where a facsimile transmission may take from 2 to 6 minutes, any interruption by the receiver substantially before the end of the transmission will consume a substantial amount of costly telephone network time, e.g., nearly 2 to 98 minutes in some instances depending upon the particular transmission mode involved.
In order to prevent a continued transmission of facsimile signals where a receiver no longer desires or no longer is capable of receiving such signals, the concept of a Procedural Interrupt Signal has been developed. The Procedural Interrupt Signal (commonly referred to as a PIS) is a signal which is generated by a receiver indicating that the receiving mode of that receiver has been interrupted for whatever reason. In accordance with CCITT Standards, the Procedural Interrupt Signal consists of a 462 Hz plus or minus 1.5 Hz signal which is generated at the receiver and applied to the communciation link. The signal is generated continuously for a 3 second minimum and is intended to be detected transmitter so as to indicate to the transmitter that the receiver no longer desires or is capable of receiving the transmission signals. It should be appreciated that the Procedural Interrupt Signal is transmitted simultaneously over the same communications link, e.g., telephone network, as the transmission signals representing dark/light variations in a document. For reasons which will now be explained, a Procedural Interrupt Signal is extremely difficult to detect at the transmitter.
In a typical telephone network communications link, the telephone network will substantially attenuate any signal which is transmitted. Since the Procedural Interrupt Signal is transmitted from the receiver to the transmitter, substantial attenuation of the Procedural Interrupt Signal when received at the transmitter may be anticipated. On the other hand, the transmission signals generated by the transmitter representing the dark/light variations in a document have not yet been attenuated by the telephone network. Accordingly, the signal level of the Procedural Interrupt Signal at the transmitter is substantially lower than the transmission signals representing the dark/light variations in the document. In some cases which are not at all typical, the Procedural Interrupt Signal at the transmitter may have a level of minus 50 dbm as compared with the transmission signals which are at a minus 9 dbm. Under these circumstances, it becomes necessary for the transmitter to detect a signal, i.e., the Procedural Interrupt Signal, which is 41 dbm below the transmitted signal, i.e., the transmission signal representing the dark/light variations in the document. Because of this very low PIS-to-noise ratio, great difficulty is incurred in attempting to detect the Procedural Interrupt Signal.